1. Field of the Invention
This invention relates to an engine speed control system for an outboard motor.
2. Description of the Related Art
When a boat powered by an outboard motor turns, accelerates or experiences certain wave conditions in the course of travel, the propeller of the outboard motor may suck in air from above the water surface and/or engine exhaust gas. When the propeller draws in air or exhaust gas, the load on the propeller decreases so that the speed of the engine rotating it rises. This may lead to overrev.
This problem is dealt with by Japanese Laid-Open Patent Application No. 2000-328996 ('996), for example, which teaches a configuration that responds to a detected engine speed exceeding a maximum speed (rev limit) by halting the operation of some of the engine cylinders, thereby lowering the engine speed below the maximum speed.
However, when considering the problem of air and exhaust gas sucked in by the propeller, it should be taken into account that the rise in engine speed owing to reduced load is accompanied by a simultaneous decrease in the thrust produced by the propeller, which gives rise to the problem of degraded power performance and steerability.
Ordinarily, therefore, the operator relies on experience to judge from the tachometer reading and engine noise that the propeller is sucking in air or exhaust gas and regulates the throttle opening finely to lower the engine speed to a level at which intake of air and/or exhaust gas no longer occurs. The period of time required to restore thrust after the propeller begins to suck in air and/or exhaust gas (i.e., the duration of degraded power performance and steerability) therefore depends on the skill of the operator.
The foregoing prior art is directed to preventing engine overrev owing to intake of air or exhaust gas and therefore cannot overcome the problem of decline in thrust owing to such intake when the engine is operating at or below the maximum speed.